FETCH_IF_OUTDATED

Fetch missing files from the server, also fetch if file present but older than the
server file.
This requires always querying the server for the last modified time of the file, thus
it adds an overhead to getting files from cache.

valueOf

Returns the enum constant of this type with the specified name.
The string must match exactly an identifier used to declare an
enum constant in this type. (Extraneous whitespace characters are
not permitted.)